This section provides a basic overview of the boot process that occurs after the GRUB boot loader finds the kernel. Understanding what happens here can help you diagnose a wide variety of boot problems, some of which you might see during a Red Hat exam.
Just a few messages after you boot a kernel from the GRUB configuration menu, Linux hands over boot responsibilities to the kernel. If you've disabled the quiet directive in the GRUB configuration file, you can watch as the messages pass quickly through the screen. To review these messages, open /var/log/dmesg or run the dmesg command.
What you see depends on the hardware and configuration of your computer. Key messages include:
The version of the kernel
Amount of recognized RAM (which does not necessarily match the actual amount of installed RAM)
CPUs (labeled as CPU 0, CPU 1, and so on)
SELinux status, if active
Kernel command line, specifying the logical volume or root filesystem label
Freeing of memory associated with the initial RAM disk (initramfs)
Hard drives and partitions (as defined by their device file names, such as /dev/sda or /dev/hda1)
Network cards, as defined by their device names, such as eth0
This file is filled with potential clues. If you've booted from the wrong kernel, you'll see it here. If Linux isn't using a partition that you've configured, you'll also see it here (indirectly). If you don't see an active network card and don't see it in the dmesg file, it may signal either a missing driver or a hardware problem with your computer.
Remember that the Red Hat exams are not hardware exams. If you identify a problem with a key hardware component, such as a network card (which cannot be solved by some Linux command), inform your instructor/ exam proctor.
While it's possible to create and install a "monolithic" kernel without drivers, that's not the way it's done on RHEL. Most kernels on modern PCs are modular. That means when you boot, kernels are loaded, followed by drivers. While many drivers are associated with hardware components, others include key software modules, such as the ext3 filesystem. You can review loaded modules using the lsmod command.